The invention relates to an elongate seismic source for use in marine operations.
To obtain information on substrata located below the sea bottom or ocean bottom, seismic sources adapted for generating a shock wave in the water are towed through the water by one or more towing vessels. The shock wave generated in the water travels in all directions, and part thereof, after having been reflected or refracted by the substrata returns to the body of water overlying the strata that are to be explored, and is subsequently picked up by hydrophones of a detector cable that is towed through the water in the neighborhood of the seismic source. This detector cable may be handled from the same vessel as from which the seismic source is being operated.
The returning reflected signals are recorded, and useful information can be gathered from such recordings to locate the position and depth of oil- or gas-containing formations.
In certain areas, a relatively large part of the shock wave generated by the seismic source does not enter the sea bottom, but is reflected by this bottom. Such reflected wave part is subsequently again reflected by the water surface, and travels for some time between the sea bottom and the water surface until at a certain moment it is picked up by by hydrophones of the detector cable. If this moment coincides with the return of the wave part reflected by the substrata, the origins of the various wave parts cannot be detected properly and the presence of such undesired multiple reflections in the recordings will lead to misinterpretation of the recordings.
By shaping the sound source such that it has a large longitudinal dimension, a kind linear sound source is obtained that is adapted for generating shock waves having a principal direction of wave propagation, which direction is confined to a narrow area on each side of a plane normal through the center of the longitudinal axis of the elongate sound source. Since a shock wave of this type travels in the preferred direction (that is the direction of the wave parts that will be reflected by the substrata and returned to the hydrophones) the influence of reflections that have passed only through the water and have been reflected frequently between the sea bottom and the water surface, is reduced.
A drawback of the known elongate sound sources is that they are very difficult to handle, and cannot be adapted to the depth of the water in which they have to operate.